The present invention is related generally to electronic devices whose functional scales are measured in nanometers, and, more particularly, to forming one-dimensional epitaxial crystals with widths and heights at the nanometer scale.
The synthesis of artificial low-dimensional structures to confine electrons has been a topic of scientific and technical interest for decades. Epitaxial deposition techniques have made possible the growth of two-dimensional quantum wells as thin as one atomic layer and xe2x80x9czeroxe2x80x9d-dimensional islands as small as a few nanometers. However, the formation of robust xe2x80x9conexe2x80x9d-dimensional nanowires with a width less than 10 nm has been a major goal that has proven difficult to achieve by either epitaxial growth or lithographic processing. Various xe2x80x9cself-assemblyxe2x80x9d techniques, in which structures form spontaneously under kinetic or thermodynamic control, have been used to grow wire-like structures directly on a semiconductor surface. Previous demonstrations include decoration of step edges on substrates, the preferential capture of adatoms onto an oriented string of dangling bonds at a surface, the growth of a lattice-matched epilayer with anisotropic surface energy, and the growth of a lattice-mismatched epitaxial layer on a low symmetry substrate with a large anisotropic stiffness. Each of these techniques depends on some type of symmetry-breaking phenomenon at the substrate surface to encourage linear growth along a preferred direction. However, they often produce wire-like structures that meander along with the steps, are terminated by single steps, have irregular shapes and sizes, and/or are not robust.
In general, an atomically flat two-dimensional epitaxial overlayer can be grown when the lattice constant of the epilayer matches that of the substrate on which it is grown. As the lattice-mismatch increases (xe2x89xa72%), the strain energy in the deposited film can be relaxed by the creation of islands of the epitaxial material. In the cases of the most often studied systems, e.g., Ge on Si(001) and InAs on GaAs(001), both the substrate surfaces and the overlayers have fourfold crystallographic symmetry with respect to the surface normal. These lattice-mismatched systems will thus limit the lateral growth in all directions on the substrate surface and produce strained islands. This suggests a strategy for the intentional epitaxial growth of linear structures: choose an overlayer material that is closely lattice matched to the substrate along one major crystallographic axis but has a significant lattice mismatch along the perpendicular axis. In principle, this should allow the unrestricted growth of the epitaxial crystal in the first direction but limit the width in the other.
A related phenomenon has been observed in the annealing behavior of Pt thin films deposited on Si(001); see, K. L. Kavanagh et al, xe2x80x9cHigh-temperature epitaxy of PtSi/Si(001), Journal of Crystal Growth, Vol. 173, pp. 393-401 (1997). Highly elongated PtSi islands formed that were at a minimum 30 nm wide and as long as 3 xcexcm. In this case, PtSi has an orthorhombic crystal structure for which the lattice mismatch with respect to Si(001) was xe2x88x926.0% along the [110]Si direction (the long axis of the island) and at least +6.3% along the perpendicular direction (the short axis). However, because of the large lattice mismatches along both directions, the sizes and shapes of the PtSi islands were quite heterogeneous.
Thus, there is a need for methods to grow one-dimensional epitaxial crystals with widths and heights at the nanometer scale, and lengths at the micrometer scale, which are aligned along selected crystallographic directions with high crystal quality.
In accordance with the present invention, self-assembled nanowires are provided, comprising nanowires of a first crystalline composition formed on a substrate of a second crystalline composition. The two crystalline materials are characterized by an asymmetric lattice mismatch, in which the first material has a close lattice match (in one major crystallographic direction within the interfacial plane formed between the two materials) with the second material and has a large lattice mismatch in all other major crystallographic directions within the interfacial plane with the second material.
The nanowires are grown by first selecting the appropriate combination of materials that fulfill the foregoing criteria. The surface of the substrate on which the nanowires are to be formed must be cleaned in order to ensure that the surface has atomically flat terraces, a regular atomic structure on terraces, and regular steps and to remove impurities. Finally, epitaxial deposition of the first crystalline material on the cleaned surface is performed, with an appropriate deposition temperature, deposition rate, deposition amount, and annealing temperature and time, thereby forming the self-assembled nanowires.
The present invention provides a means to grow one-dimensional epitaxial crystals with widths and heights at the nanometer scale, and lengths at the micrometer scale, which are aligned along certain crystallographic directions with high crystal quality. Further, the present invention avoids traditional lithography-deposition-etching methods, minimizes defects formed during the normal lithography process and environmental toxic chemicals usage, simplifies the manufacturing processes, and allows self-assembled formation of high-quality one-dimensional nano-wires over large areas.